This grant proposal is to study cellular immune mechanisms in gram-negative sepsis. Gram-negative sepsis is a major infectious disease problem associated with considerable mortality despite intensive antimicrobial therapy. Despite extensive work designed to test immunotherapeutic approaches to this problem, host defense to gram-negative sepsis is poorly understood. Conventional teaching about immunity to gram-negative rods centers on the role of protective antibody. Advocates of immunotherapy to gram-negative sepsis have concentrated on administration of antisera. Gram-negative aerobes contain a surface lipopolysaccharide (LPS) or endotoxin, the core region of which is similar chemically among a variety of virulent strains. A rough mutant E. coli (J5) which lacks the enzyme necessary to attach antigen-specific "O" side-chains to the core LPS has been employed after heat-killing as a vaccine to generate specific antisera in mice and humans. Trials of antisera administration in animal models and man have led to reduced mortality, but attempts to correlate beneficial effects with antibody titers have not been successful. Based on suggestive preliminary experiments, specific splenic T cells or factor(s) derived from these cells appear to play a role in protection against gram-negative sepsis. This proposal concerns a detailed study of this mechanism of immunity, specifically: 1) to confirm that cells and particularly T cells are implicated, 2) to determine whether antibody is the active constituent of protective serum, 3) to define the protective T cell subsets in mice and demonstrate whether a cellular lymphokine is important in protection, 4) to characterize partially the lymphokine and determine whether intact T cells are necessary for activity, 5) to assess the antigen-specificity or broad protective capacity of cells or factor and 6) to examine in vitro cell, factor, serum and specific antibody interactions in killing E. coli. A role for cellular immune mechanisms in the critical disease state of gram-negative sepsis expands concepts of host-defense. The potential importance of this work lies in the definition of an immune mechanism which could tailor immunotherapeutic approaches. The potential exists to develop a vaccine of cellular origin which might be more potent than the antisera currently produced. Definition of immune mechanisms to gram-negative sepsis also offers promise in the development of a more detailed understanding of the pathogenesis and prevention of a significant disease state.